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Expanding Universe

  1. Oct 29, 2014 #1
    How one can say that the space is expanding by just verifying that the galaxies are speeding away from each other?
  2. jcsd
  3. Oct 29, 2014 #2


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  4. Oct 29, 2014 #3


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    Galaxies speeding away from each other is what it MEANS when you say that the universe is expanding. I guess I don't understand your question.
  5. Oct 30, 2014 #4


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    We can't. Not by just looking at galaxies. There are several key pieces of evidence that add up, namely the CMB, the observed recession of galaxies, the homogeneous and isotropic nature of the universe at the largest scales, the prediction of primordial elements created in the early universe, and more.
  6. Oct 30, 2014 #5
    We can predict that the universe must be expanding by looking at the sky. The sky is only as hot and bright as the Sun's surface where the Sun is located. If the universe was static and eternal, it would be as hot and bright in every direction, all star's light would sum without energy loss during travel. (Olber's paradox; http://en.wikipedia.org/wiki/Olbers'_paradox )

    When we use that cosmological redshift to predict the same but seen as the average apparent velocity of galaxies, which is receeding, we have to add the constraint that the universe is homogeneous and isotropic. That constraint is given by the simplest universes that general relativity produces, so called FRW universes, and that is the kind of universe we live in. Then we know that it happens irrespective of the observer's location, and hence is a result of spacetime expansion rather than an unlikely conspiracy among actually traveling galaxies (that they don't like our breath, say, and the farther out they are the faster they want to escape it :L).

    But the evidence is much firmer than that as Drakkith describes. Going back to redshifts, there is also a natural velocity "noise" added on the cosmological expansion, signifying the gravitational effects of the superclusters like our own Laniakea [ http://www.nature.com/news/earth-s-new-address-solar-system-milky-way-laniakea-1.15819 ]. So we know they are separate effects.

    And to top that the current inflationary cosmology makes the expansion inevitable. You can't predict a self-consistent universe, consistency among our observations, without it.
    Last edited: Oct 30, 2014
  7. Oct 30, 2014 #6
    The expansion of space is not a reference to the speed of galaxies but to the consistencies in red shift of element spectra from distant galaxies. Because of the red shift being proportionally the same is every direction the red shift comunicates that space itself is expanding. If space was not expanding the proportional redshift would not occur. However red/blue shift is used to determined relative velocities to earth.
  8. Oct 30, 2014 #7


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    Actually, I think it IS, since "space" isn't really doing anything ... things are just getting farther apart in the framework of space-time. Google "metric expansion" for more. I know it's common to TALK as though space itself were expanding, but the consensus seems to be that space is not a thing that can bend/warp/expand, etc., it's just a part of the space-time framework in which things happen.
  9. Oct 30, 2014 #8
    (Assuming Big Bang cosmology) Before the BB there was no space - time. "The center of expansion today is every point in the universe because every point was the same point in the beginning". (Allen Sandage)

    Space is doing much more than expanding, it is full of virtual particles. Also space does bend due to gravity.

    So, as mentioned in this forum - the big bang did not happen as a place in space but as a place in time.

    There is no space outside of space-time. Theoretically speaking in BB cosmology.
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